The present invention relates to an improved process for preparing 2,3,5,6-tetrachloropyridine from pentachloropyridine(PCP).
2,3,5,6-Tetrachloropyridine is a valuable commercial product which can be used for the production of insecticidal formulations. Furthermore, 2,3,5,6-tetrachloropyridine is used as an intermediate for producing herbicidally effective .alpha.-[4-(3',5',6'-trichloropyrid-2'-yloxy)-phenoxy]-alkanecarboxylic acids and derivatives thereof. The production and use of such .alpha.-[4-(3',5',6'-trichloropyrid-2'-yloxy)-phenoxy]-alkanecarboxylic acids and derivatives thereof are described, for example, in the U.S. Pat. No. 4,133,675.
Methods are known for preparing 2,3,5,6-tetrachloropyridine from pentachloropyridine. U.S. Pat. No. 3,993,654 teaches a process of preparing tetrachloropyridine by reacting pentachloropyridine, zinc and hydrochloric acid in an aqueous medium at a temperature of at least 110.degree. C. under at least autogeneous pressure. U.S. Pat. No. 4,259,495 teaches a process of preparing 2,3,5,6-tetrachloropyridine by reacting pentachloropyridine with zinc in an alkanephosphoric acid dialkyl ester solvent in the presence of an ammonium salt. Although achieving good yields, this process has the serious disadvantage that it is difficult to separate the alkanephosphoric acid dialkyl ester solvent from the desired 2,3,5,6-tetrachloropyridine product without elaborate or sophisticated separation apparatus. Another disadvantage is that the high boiling point of the alkanephosphoric acid dialkyl ester solvent makes this solvent difficult to recycle in industrial processes, requiring incineration or land disposal of an otherwise expensive and polluting solvent. U.S. Pat. No. 4,259,495 also discloses that the process employs 1.2 to 1.6 gram-atoms of zinc per mole of pentachloropyridine. Zinc is one of the most expensive reactants used in the process of making 2,3,5,6-tetrachloropyridine from pentachloropyridine. It would clearly be desirable to carry out a process which utilizes a solvent which can be readily recycled and which utilizes zinc even more efficiently than the process disclosed in U.S. Pat. No. 4,259,495 in order to reduce operating expenses involved with waste disposal and the cost of zinc.